1. Field of the Invention
This invention relates generally to communication systems and, more particularly, to power regulation in multi-carrier communication channels.
2. Description of the Related Art
Code Division Multiple Access (CDMA) technology is often used in communication systems to transmit video, voice or computer data in a sequence of signal pulses. CDMA provides a higher capacity transfer of data when compared to other technologies, such as Time Division Multiple Access (TDMA) or Frequency Division Multiple Access (FDMA). TDMA typically transmits data from multiple users in the same frequency band at different times, while FDMA typically transmits data from multiple users in different frequency bands at the same time. Hence, in TDMA and FDMA, the data from a particular user can be detected at a unique time or frequency band, respectively.
CDMA can provide a higher data transfer rate because CDMA transmits multiple channels of information onto a given frequency band through the use of digital encoding. The digital encoding can include unique user assigned codes to distinguish the different users. Unlike TDMA, multiple CDMA codes and multiple FDMA frequencies can be active at the same time, which increases the potential magnitude of rare large amplitude peaks. The large amplitude peaks can be caused by constructive interference between signals transmitted on different frequencies. As more signals are added together, the magnitude and likelihood of large amplitude peaks from constructive interference increases. More information regarding peak power regulation in communication systems can be found in U.S. Pat. No. 6,236,864.
Multi-channel CDMA and FDMA systems typically include linear power amplifiers. However, linear power amplifiers capable of reproducing large amplitude peaks are expensive to manufacture and operate. In addition, when a linear amplifier saturates because of a large amplitude peak, it can produce broadband distortions which can persist after the large amplitude signal has passed.
Some prior art approaches increase the dynamic range of the power amplifier to better accommodate large amplitude peaks. The dynamic range typically refers to the range of signal amplitudes capable of being processed without distorting the signal or saturating the amplifier or other components included in the system. However, this is costly and increases power consumption and cooling requirements.
Other prior art approaches limit or clip the signal amplitude at various points in the transmission channel of the communication system. The limiting can be done early or late in the transmission channel or at many points along the transmission channel. Limiting, however, has many disadvantages. For example, limiting can cause broadband distortions which typically need to be filtered to meet emissions specifications. Limiting late in the transmission channel produces better signal quality, but poor spectral confinement which leads to broadband frequency distortions which must be filtered. Another disadvantage is that it is difficult to balance the limiting when clipping along the length of the channel. Further, the filters are relatively expensive and tend to partially restore the original peaks. In addition, the remaining distortions are distributed according to the frequency response of the filter. Consequently, there is a need for a communication system that can process large amplitude peaks without causing saturation or distortion.